(1) Field of the Invention
The present invention is directed to a new class of luminescent gold(III) compounds containing a tridentate ligand and having at least one strong σ-donating group. Such compounds can be used as light-emitting material in phosphorescence based organic light-emitting devices (OLEDs), wherein different colors can be obtained by varying the applied DC voltage or the dopant concentration of said compounds.
(2) Description of Related Art Including Information Disclosed Under 37 C.F.R. 1.97 and 1.98.
The market for flat-panel displays has attracted considerable attention in connection with the development of electroluminescent materials. The electroluminescence materials used are generally categorized into conjugated polymers or low-molar-mass small molecules. OLEDs are among the most important candidates for the use of electroluminescent materials in commercial flat-panel displays because OLEDs possess the advantages of robustness, ease of fabrication and color tuning, wide viewing angle, high brightness and contrast ratios, low turn-on voltage and low energy consumption.
A typical OLED structure is composed of a thin film of organic material sandwiched between a transparent conductor such as indium tin oxide (ITO), and a vapor deposited metal cathode. Upon applying an electrical potential, excitons are formed by the recombination of the holes and electrons, injected from the ITO electrode and metal cathode, respectively. Electroluminescence is generated in the organic material from the radiative relaxation of excitons. Higher performance of the device can be achieved by using multiple organic layers for separation of hole and electron transporting layers.
Although electroluminescence from organic polymers was initially reported years ago [Kaneto, K.; Yoshino, K.; Koa, K.; Inuishi, Y. Jpn. J. Appl. Phys. 18, 1023 (1974)], it was only after the report on yellow-green electroluminescence from poly(p-phenylenevinylene) (PPV) that light-emitting polymers and OLEDs have received much attention [Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, N.; Friend, R. H.; Burn, P. L.; Holmes, A. B., Nature 347, 539 (1990)]. Later on, similar studies were reported by using PPV derivatives as the light-emitting polymers [Braun, D.; Heeger, A. J., Appl. Phys. Lett. 58, 1982 (1991)]. Since then a number of new electroluminescent polymers have been investigated for improved properties.
Electroluminescence of organic materials was discovered in anthracene crystals immersed in liquid electrolyte in 1965 [Helfrich, W.; Schneider, W. G. Phys. Rev. Lett. 14, 229 (1965)]. Although lower operating voltages could be achieved by using a thin film of anthracene as well as solid electrodes, very low efficiency of such a single-layer device was encountered. High-performance green electroluminescence from an organic small molecule, aluminum tris(quinolate) (Alq3), was first reported in 1987 [Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 51, 913 (1987)]. A double-layer OLED with high efficiency and low operating voltage was described, in which Alq3 was utilized both as emitting layer and electron transporting layer. Subsequent modifications of the device with triple-layer structure gave better performance with higher efficiency.
Some improvements in OLED efficiencies have been achieved by using phosphorescent material to generate light emission from both singlet and triplet excitons. One approach, particularly for small-molecule OLEDs, is to harvest triplet excitons efficiently through incorporation of heavy metal centers, which would increase spin-orbit coupling and hence intersystem crossing into the triplet state. In 1998, Baldo et al. demonstrated a phosphorescence electroluminescence device with high quantum efficiency by using platinum(II) 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (PtOEP) as a dye [Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikow, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 395, 151 (1998)]. A multilayer device in which the emitting layer of Alq3 is doped with PtOEP showed a strong emission at 650 nm attributed to the triplet excitons of PtOEP. Cyclometalated iridium(III) is known to show phosphorescence and is another class of materials used for high efficiency OLEDs. Baldo et al. reported the use of fac-tri(2-phenylpyridine)iridium(III) [Ir(ppy)3] as phosphorescence emitting material which was doped in 4,4′-N,N′-dicarbazole-biphenyl (CBP) as a host in an OLED to give high quantum efficiency [Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 75, 4 (1999)]. In addition, fac-tri(2-phenylpyridine)iridium(III) [Ir(ppy)3] was used as phosphorescence sensitizer for high efficiency fluorescent OLED [Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nature, 403, 750 (2000)]. Using the concept of a phosphorescence emitter with a higher population of excitions, very high efficiency red fluorescence from [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[II]quinolizin-9-yl)-ethenyl]-4H-pyran-4-ylidene]propanedinitrile (DCM2) was found in a multilayer OLED composed of Ir(ppy)3 and DCM2 dopant layers. In a sensitization process, energy is transferred from Ir(ppy)3 to DCM2 to give such high efficiency fluorescence.
Apart from the enhancement of the emission efficiency, the ability to bring about a variation in the emission color would be important. Most of the common approaches involve the use of different light-emitting materials or multi-component blended mixtures of light-emitting materials with different emission characteristics for color tuning. Examples that employ a single light-emitting material as dopant to generate more than one emission color have been rare. Recent studies have shown that different emission colors from a single emissive dopant could be generated by using phosphorescent material through a change in the direction of the bias or in the dopant concentration. Welter et al. reported the fabrication of a simple OLED consisting of semiconducting polymer PPV and phosphorescent ruthenium polypyridine dopant [Welter, S.; Krunner, K.; Hofstraat, J. W.; De Cola, D. Nature, 421, 54 (2003)]. At forward bias, red emission from the excited state of the phosphorescent ruthenium polypyridine dopant was observed, while the OLED emitted a green emission at reverse bias in that the lowest excited singlet state of PPV was populated. Adamovich et al. reported the use of a series of phosphorescent platinum(II) [2-(4,6-difluorophenyl)pyridinato-N,C2′] β-diketonates as single emissive dopant in OLED [Adamovich, V.; Brooks, J.; Tamayo, A.; Alexander, A. M.; Djurovich, P. R.; D'Andrade, B. W.; Adachi, C.; Forrest, S. R.; Thompson, M. E. New. J. Chem. 26, 1171 (2002)]. Both blue emission from the monomeric species and orange emission from the aggregates were observed in such OLED and the relative intensity of the orange emission increases as the doping level is increased. As a result, the electroluminescence color can be tuned by changing the dopant concentration, and white illumination sources of an OLED can be obtained in a doping concentration with equal intensities of the monomeric and aggregate bands. In both cases, the change of electroluminescence color in OLED can be accomplished upon a variation of the external stimulus or fabrication conditions while keeping the light-emitting material the same.
Despite recent interest in electrophosphorescent materials, in particular metal complexs with heavy metal centers, most of the work has been focused on the use of iridium(III), platinum(II) and ruthenium(II), while other metal centers have been relatively less extensively explored. In contrast to the isoelectronic platinum(II) compounds which are known to show rich luminescence properties, very few examples of luminescent gold(III) compounds have been reported, probably due to the presence of low-energy d-d ligand field (LF) states and the electrophilicity of the gold(III) metal center. The introduction of strong σ-donating ligands into gold(III) compounds to enhance the luminescence properties as a result of the enlargement of d-d splitting has been considered. Yam et al. first demonstrated that gold(III) aryl compounds are photo-stable and are capable of displaying interesting photoluminescence properties which occur even at room temperature [Yam, V. W. W.; Choi, S. W. K.; Lai, T. F.; Lee, W. K. J. Chem. Soc., Dalton Trans. 1001(1993)]. Another interesting donor ligand is the alkynyl group. But despite the fact that a number of gold(I) alkynyls are known and have been shown to exhibit interesting luminescence properties, the chemistry of gold(III) alkynyls has been essentially ignored, except for a brief report on the synthesis of an alkynylgold(III) compound of 6-benzyl-2,2′-bipyridine in the literature [Cinellu, M. A.; Minghetti, G.; Pinna, M. V.; Stoccoro, S.; Zucca, A.; Manassero, M. J. Chem. Soc. Dalton Trans. 2823 (1999)], but their luminescence behaviour has remained totally unexplored. The present inventors have described herein the design, synthesis and photoluminescence behaviors of luminescent gold(III) compounds with a tridentate ligand and at least one strong σ-donating group, and the use of such compounds as electrophosphorescent material in OLEDs to give strong electroluminescence with high efficiency.